Growth spurts are really tough on teenage pitchers trying to maintain or develop control. Usually the repetition of throwing pitch after pitch improves control, teaching your brain and neural system the speed and timing necessary. But growth spurts disrupt this cycle especially where boys lack functional strength.
Of course control issues can be linked to mechanical problems, but mechanics can change as the body grows. So this to me is more appropriate for those late HS and college guys who are done growing and still fighting control issues. Those 8th/9th grade years in particular seem to have some significant growth spurts. Developing core/functional strength is key for kids in those high growth spurt age ranges.
Be patient, keep throwing, stay mentally strong and ignore the haters. My LHP grew to be over 6'3" (incl 9" over a 2-yr stretch) and struggled bad with control at times. Imagine if he quit because he couldn't throw strikes when he was 5'9", getting harsh criticism from coaches, teammates (and himself!) and just starting to grow. He's now signed to D1. His HS teammate is now about 6'5" and did give up pitching in 9th grade because he lacked control. Picked it back up in 11th grade in the upper 80s, worked hard in the weight room, and currently he is throwing mid 90s and fielding calls from MLB teams daily, likely drafted in a couple weeks.
For immediate results, slowing down will help. But I think long-term this is poor advice. Velo is king to progress to the next level. Of course control needs to be there if your son wants innings NOW, but accept that growth spurts will put more demands on re-teaching your brain and nervous system, and that just takes time (and lots of practice, done safely).